1. Field of the Invention
The solution according to an embodiment of the present invention relates to the electronics field. More specifically, an embodiment relates to the control of a resonant switching system.
2. Discussion of the Related Art
Resonant switching systems are commonly used in various applications for driving electronic loads; a typical example is an electronic ballast for controlling the brightness of fluorescent lamps.
In general, a switching system of a ballast is based on two power transistors in a half-bridge configuration, which are switched on and off alternatively by proper control signals generated trough an oscillator, in this way, it is possible to obtain a power supply alternated voltage of the lamp at a desired frequency from a power supply direct voltage of the switching system (obtained from a mains alternated voltage at fixed frequency). The lamp is inserted in an RLC resonant circuit, which therefore has a reactance that is equal to zero at a resonant frequency thereof and increases moving away from it (becoming of the capacitive type or of the inductive type when the frequency decreases or increases, respectively); as a consequence, it is possible to control the reactance of the RLC circuit (and thus the current crossing the lamp) by properly varying the frequency of the oscillator (and thus the frequency of the alternated voltage). This allows controlling the brightness of the lamp in a simple and effective way, with reduced power losses (since the limitation of the current through the lamp is obtained without the use of any resistor).
The ballast can control the brightness of the lamp with either an open-loop or a closed-loop structure. In particular, U.S. Pat. No. 6,002,214 describes a ballast with a closed-loop control system based on the monitoring of the phase of the current of the resonant circuit with respect to its voltage (proportional to the power supplied to the lamp). The phase of the current is compared with the phase of the oscillator, so as to modify it accordingly for locking the phases of the current and of the oscillator to each other. For such purpose, there is proposed to detect an instant of zero-crossing by the current. Such result is obtained by comparing the voltage across a resistor in series with one of the transistors with a reference voltage, so as to obtain a pulse in correspondence of positive values of such voltage; the pulse and the command signal of the transistor are supplied to an AND gate for limiting the pulse to the period wherein such transistor is turned on. At the same time, a signal provided by the oscillator is compared with the same reference voltage, so as to obtain an opposed pulse in correspondence of negative values of such signal. The two pulses thus obtained are applied to an AND gate for obtaining an error signal having a length equal to their phase difference. The error signal charges a capacitor of the oscillator by an amount proportional to its length, so as to increase the frequency of the oscillator; on the contrary, when the two pulses are in phase, the capacitor is slightly discharged, so as to decrease the frequency of the oscillator.
In general, the transistors of the switching system are preferably turned on in a “soft” mode—that is, with a voltage substantially zero at their terminals. This allows limiting the power losses during the switching (with the possibility of reducing or even eliminating the cooling fins of the transistors); as a consequence, it is possible to increase the efficiency of the switching system, and to use smaller transistors. In any case, the area occupied by the transistors reduces, with the possibility of a more rational accommodation thereof. Moreover, such operating mode increases the reliability of the switching system. On the contrary, a “hard” turning on of the transistors—that is, with a voltage not null at their terminals—causes a significant increase of the power being dissipated during the switching and, in extreme cases, it may cause the breaking thereof.
Moreover, it is also preferable that the resonant circuit operates in the inductive mode. In fact, when the resonant circuit operates in the capacitive mode the current leads the voltage; therefore, the turning on of the transistors always occurs after the current has already started increasing the voltage at their terminals (so that such turning on cannot occur in the soft mode, with the drawbacks pointed out above). In addition, since the possible control system of the ballast acts on the oscillator frequency, it follows that the open loop gain depends on the derivative of the impedance of the resonant circuit with respect to the frequency. However, such derivative is negative in the inductive operation mode (where the gain decreases with the increase of the frequency moving away from the resonance frequency) and positive in the capacitive operation mode (wherein the gain increases with the increase of the frequency towards the resonance frequency). Therefore, passing from the inductive operation mode to the capacitive operation mode the feedback of the closed loop changes sign, turning a feedback to being designed to be negative into a positive feedback (with subsequent risk of instability).
Another problem of the switching systems with closed-loop control systems occurs when the mains voltage decreases below a nominal value thereof (situation that may occur in certain locations for relatively long periods, even of several hours), with a consequent decrease in the obtained direct voltage and thus in the alternated voltage being supplied to the lamp. In such condition, the control system decreases the frequency (towards the resonance frequency in the inductive operation mode) for increasing the current and thus maintaining a constant brightness of the lamp (notwithstanding the decrease of the alternate voltage). This may bring the resonant circuit to operate in the capacitive mode, with the same above-listed drawbacks.
In order to avoid such drawbacks, the ballasts are sized so as to ensure their correct operation in most practical conditions (for example, by imposing a lower limit to the frequency). However, this does not allow using the lamp optimally, since the limits imposed by the worst operating conditions (including the inevitable spreads of the operating characteristics—for example, of the lamp, of the ballast, of the components and of the mains voltage) adversely affect the normal operating conditions. Moreover, this does not allow ensuring the correct operation of the lamp in emergency conditions.